Rheumatic diseases are among the most common diseases in Germany. A diagnostic test which allows to attribute the medical condition of a particular patient to harmless muscle tenseness, arthrosis or to the most frequent and severe of said diseases, rheumatoid arthritis (RA) is not known to date.
Rheumatoid arthritis is an autoimmune disease in which the defence mechanisms of the human body erroneously regard endogenous joint cartilage as foreign and hostile and attack said cartilage. Approximately 1 out of 100 humans in western European countries suffers from rheumatoid arthritis. The disease progresses very rapidly in the first few months.
An important key strategy in modern rheumatology is therefore the early onset of treatment e.g. with drugs which may positively modify the course of the disease. Numerous clinical studies have shown that very good therapeutic success and response rates can be achieved using suitable active compounds, for example TNF antagonists, if said compounds are used in patients already in the early stages of the disease. Rheumatologists try to utilize the narrow time window between the onset of the disease and the occurrence of structural joint damage. To date, however, no reliable and sensitive detection of rheumatoid arthritis within said time window has been disclosed in the prior art.
Rheumatoid arthritis is diagnosed according to the classification criteria of the ACR (American College of Rheumatology). According to the criteria of the ACR, the rheumatoid factor is currently the fundamental serological indicator for diagnosing rheumatoid arthritis (RA). Rheumatoid factors are a subgroup of immunoglobulins which are distinguished by immunological cross reaction to the Fc region of immunoglobulin G (IgG). However, the presence of a rheumatoid factor is not limited to disorders of the rheumatic type (therefore necessitating a differential-diagnostic evidence), and rheumatoid factors are also found in the serum of patients suffering from infectious diseases, hyperglobulinemias, lymphoproliferative B cell disorders, and generally in the older population.
Usually, elevated concentrations of rheumatoid factors are associated with a more severe course of the disease. Said concentrations do not correlate with the degree of activity and the therapeutic success. A sensitive and specific prognosis of the onset of rheumatoid arthritis cannot be made on the basis of the concentration of rheumatoid factors. Healthy persons may have an elevated rheumatoid factor concentration without being affected, while some patients without elevated rheumatoid factor concentrations suffer from a very aggressive form of rheumatoid arthritis.
Other serological markers such as anti-citrulline antibody (CCP) or the initial HAQ score which is used to assess abilities in daily life or X-ray or computer tomography (CT) imaging provide only little information on the early form and are, by themselves, not meaningful enough in order to be able to assess the prognosis of the patient.
In order to optimise the existing classification criteria of the ACR, the American College of Rheumatology proposes seven classification criteria which indicate a poor prognosis:    1. morning stiffness of the joints lasting more than one hour,    2. arthritis of three or more joints,    3. inflammation of at least three joint areas at the same time,    4. hand joints or finger joints are likewise affected,    5. bilateral tenderness of metacarpophalangeal joints to pressure,    6. erosions on radiographs, and    7. detection of special rheumatoid factors and anti-perinuclear factor positivity (APF).
Autoantibodies to the “anti-perinuclear factor” were first described by Young et al. for patients having rheumatoid arthritis (Young, B. J. J. et al., Antikeratin antibodies in rheumatoid arthritis, B.M.J., 2 (1979), 97-99). Owing to their specific reaction to the keratinous epithelium of the stratum corneum on rat oesophagus sections, keratin has long been considered to be the corresponding antigen (Vincent, C. H. et al.; High diagnostic value in rheumatoid arthritis of antibodies to the stratum corneum of rat oesophagus epithelium, so-called “antikeratin antibodies”, Ann. Rheumat. Dis. 48 (1989), 712-722). For this reason, the antibodies are even today referred to as antikeratin antibodies (AKAs) (Vincent, C. H. et al, Natural IgG to Epidermal Cytokeratins vs IgG to the Stratum Corneum of the Rat Oesophagus Epithelium, so-called “Antikeratin Antibodies”, in Rheumatoid Arthritis and other Rheumatic Diseases; J. of Autoimmunity 4 (1991), 493-505; F'aimela, L. et al., Antikeratin antibodies: diagnostic and prognostic markers for early rheumatoid Arthritis, Ann. Rheumat. Dis., 51 (1992) 743-746).
Later studies have demonstrated that at least some AKA or APF reactive sera also exhibit anti-filaggrin antibody activity. Thus the basic protein filaggrin has been identified as a target antigen. The 40 kDa protein aggregates cytokeratin filaments and assists in forming the intracellular fiber matrix of the keratinous cells (Simon, M. et al., The Cytokeratin Filament-Aggregating Protein Filaggrin is the Target of the So-called “Antikeratin Antibodies”, Autoantibodies Specific for Rheumatoid Arthritis, J. Clin. Invest., 92 (1993), 1387-93).
Since sera containing APFs, AKAs and anti-filaggrin antibodies react in the same way, these antibody systems appear to be identical. Anti-filaggrin antibodies of the IgG type which have a specificity of more than 99% are a highly specific marker for rheumatoid arthritis. Several studies found positive correlations with respect to severity and activity of the disease. Anti-filaggrin antibodies do not correlate with age, sex or duration of the disease.
Using currently customary methods, however, said antibodies can be found in the serum of only approx. 40% of RA cases.
It was therefore the object of the present invention to provide novel polypeptides for detecting antibodies associated with rheumatoid arthritis, that allow for an improved diagnosis of RA.